This invention relates to vocational trucks, such as cement mixers, and particularly to systems and methods for controlling the engine of the truck. More specifically, the invention relates to systems for controlling the engine speed in different operational modes of the vocational truck.
Most vocational trucks are driven by internal combustion engines, such as diesel engines. One such vocational truck is the well-known mobile cement mixer, that carries a charge of concrete from an aggregate batch plant to a remote job site.
For most construction sites, it is customary to have the concrete delivered by these mobile cement mixers. The vehicles are loaded with sand, stone, cement and water, in the correct proportions to meet industry-wide concrete specifications. These concrete specifications typically require that the concrete on arrival at the job site be guaranteed to achieve a minimum specified strength ninety-nine percent (99%) of the time.
Once the mixing drum of the mobile cement mixer has been charged with all the necessary ingredients, the mixing cycle can commence. In the instances in which a dry batch is being hauled by the mixer, some nominal agitation of the dry mix occurs before and during transit. The critical mixing occurs when water is added to the dry batch. In some cases, water can be added at the batch facility, so that more significant agitation or mixing of the wet batch must be accomplished during transit to the job site.
The strength of the concrete when ultimately set, and its workability at the job site, are critically dependent on the mixing regime that is followed. Certain standards have been developed and are generally adhered to in the industry. Once such standard is the Truck Mixer Manufacturer's Bureau (TMMB) standard that provides the following recommended criteria for the mixing of a full load of concrete:
1. Mixing turns: 70-100 turns at 6-18 rpm; PA1 2. On addition of further water, a minimum of 30 additional turns at mixing speed; and PA1 3. Holding or agitation turns at no greater than 6 rpm for no more than 300 total turns including mixing turns.
Once the concrete has been properly and consistently mixed according to the above protocol, it is also important to maintain a minimum degree of further agitation to prevent separation of the aggregate material. Preferably, this agitation occurs at about 1.5-2.5 rpm. Any greater rotational speed can accelerate the setting of the concrete by overworking.
Traditionally, responsibility for controlling the rate and duration of rotation of the mixing drum has been left to the vehicle operator. The vehicle operator can control the speed and duration of the rotation of the mixing drum by controlling the mixing drum drive system. Typically, this system includes a hydraulic motor that rotates the drum, and a variable-stroke hydraulic pump that provides hydraulic fluid to the motor. The vehicle operator can control the speed of rotation of the mixing drum by operation of a stroke control arm on the hydraulic pump. Hydraulic motors and control systems of this type are well known in the art. Of course, other devices that permit controllable rotation of the mixing drum are contemplated by the present invention.
The mixing drum speed is a function of the vehicle engines speed. In vocational truck applications, the engine is provided with a power take-off (PTO) that diverts engine power from the driven wheels to an auxiliary driven component. In the case of a mobile cement mixer, the driven components is the hydraulic pump and motor power train driving the mixing drum.
In a typical scenario, once the vehicle mixing drum has been filled with a full charge of ingredients, the operator will set the hydraulic pump to obtain the maximum rate of drum rotations within the recommended mixing speed range. This mixing step occurs while the vehicle is at the aggregate batch plant since it is dangerous to drive the vehicle while the drum is rotating at a high speed.
Typically, the vehicle engine will be operated at a high rpm level to drive the PTO in the mixing mode. This high rpm is significantly higher than the usual idle speed when the vehicle is stationary, in order to provide adequate power and/or to drive the mixing drum.
On departure, the vehicle operator will set the stroke to the agitation speed. At this setting, the rate of rotation of the drum depends upon the vehicle engine speed, which can lead to significantly variability in the agitation speed of the drum.
One problem that is encountered with cement mixers is caused by the abrasive effect of the aggregate mixture. More specifically, the concrete materials cause significant wear on the interior surface and mixing vanes of the mixing drum. The amount of wear and damage is a function of the speed of rotation of the mixing drum and ultimately the abrasive aggregate contained therein. In addition, excessive rotation of the mixing drum at mixing speeds increases the fuel usage for the engine, leading to a serious drop in fuel economy for the vehicle.
Consequently, there is a tradeoff between operating the mixing drum for ideal mixing of the aggregate, and the damage to the mixing drum and decreased fuel economy of the engine. Thus far, no engine control system has been developed that optimizes both sides of this tradeoff. More particularly, no system exists that automatically controls the vehicle engine to minimize the amount of time that the engine is running at its high rpm PTO output speed, while insuring that the aggregate within the mixing drum is fully mixed.